The ubiquitous process of protein phosphorylation is central to signal transduction and regulation in living organisms. By catalyzing transfer of the γ-phosphoryl group of ATP to the side chains of serine, threonine, and/or tyrosine, protein kinases play an important role in regulating many aspects of cellular function in eukaryotes, including proliferation, cell cycle, metabolism, transcription, and apoptosis. Not surprisingly, protein kinases have also emerged as attractive targets for drug discovery, since many are associated with a wide variety of diseases, from cancer to inflammation. Thus, tools that allow for facile monitoring of kinase activity are in great demand in both pharmaceutical and academic settings.